1. Field of the Invention
The present invention relates to an optical scanning apparatus to be employed in an image reading apparatus for reading an image formed on an object of reading by optically scanning the object of reading.
2. Description of Related Art
An image reading apparatus proposed in U.S. Pat. application Ser. No. 08/253,321 filed Jun. 3, 1994 by the applicant of the present patent application, now U.S. Pat. No. 5,610,652, copies an image formed on a document and produces a monochromatic copy of the image. This previously proposed image reading apparatus deflects an image reading light beam emitted by a monochromatic light source by a polygonal rotating mirror to scan an image formed on a document. The light beam is reflected by the image surface of the document and the reflected light beam falls on a light receiving device, such as a photodetector. Image information about the image formed on the document is obtained on the basis of output signals provided by the light receiving device.
Generally, the image information is obtained on the basis of the output signals of the light receiving device by a method that compares the intensity of the light beam emitted by the image reading light source and the intensity of the reflected light beam reflected by the image surface of the document to determine the attenuation of the image reading light beam, and determines the density of the image on the basis of the attenuation.
When producing a copy of an image formed on a document by a copying machine provided with an image reading device of this type, an image on one page of the document is read by the aforesaid method to obtain image information about the image, and the image information is stored in a storage device. Then, a light beam emitted by a recording light source (generally, a semiconductor laser) is modulated according to the image information stored in the storage device, the modulated light beam is deflected by a polygonal rotating mirror onto the surface of a photoconductive body for scanning to form an electrostatic latent image corresponding to the image information on the surface of the photoconductive body. Toner charged in a polarity opposite to that of charges on the photoconductive body sticks to portions of the surface of the photoconductive body forming the electrostatic latent image to form a toner image on the photoconductive body, and then the toner image is transferred to a recording sheet to produce a monochromatic image on the recording sheet.
When the storage device has a large storage capacity, a copying operation may be started after reading all images on the document and storing image information about those images in the storage device.
However, since the prior art copying machine uses the monochromatic light source for reading the images on the document, the copying machine is unable to read portions of multicolor images of some colors with the light beam emitted by the monochromatic light source. Consequently, the thus formed monochromic image is missing portions of the original image of the colors which cannot be read by using the monochromatic light beam and hence the original image cannot be copied with high fidelity.
For instance, when a prevalently used semiconductor laser that emits a red light beam of about 670 nm in wavelength is used for reading a multicolor image, the total reflection of the red light beam occurs in red portions of the multicolor image and, consequently, image information obtained by the foregoing method is missing image information about the red portions.
Although such a problem may be solved by using a white light source, such as a halogen lamp, for reading all portions of colors of an image, a light beam emitted by the white light source, as compared with a laser beam, is unsatisfactory in directivity and focusing capability and hence the resolution of image information obtained by using the light beam emitted by the white light source is low. Since the light beam emitted by the white light source includes radiations of wavelengths outside the visible region, such as infrared radiations, a filter, such as an infrared ray absorbent filter, must be placed on the light receiving surface of a light receiving device to cut the reflected radiations of wavelengths outside the visible region. A copying machine provided with such a filter is complex in structure and expensive.